This invention relates to a MOS ROM and more particularly to a structure and method of programming a MOS ROM.
One device in the hierarchy of semiconductor memories is the Metal-Oxide-Semiconductor Read Only Memory or MOS ROM. The device is a fixed memory programmed during fabrication and is typically fabricated and programmed to incorporate a particular user program.
In the present technology, most ROMs are programmed at the active area patterning step. By this method no active area is provided for devices in those locations designed to incorporate an off-state. Instead of providing an active area as for a normal device, thick field oxide is grown in that location. Active area programming reliably provides a ROM having the desired programming, but has the serious drawback that the active area programming is done at a process step very early in the total fabrication process. It is preferable, however, to do the programming at as late a step in the process as is possible.
Because a number of ROM devices are identical except for the information stored at the programming step, unprogrammed devices can be processed up to the programming step and stockpiled while waiting for orders for particular custom programs. Upon receipt of a customer order the partially completed devices can be programmed, quickly completed, and shipped to the customer. The stockpiling of partially completed devices permits the more rapid filling of customer orders. The later in the process the programming can be accomplished, the shorter the turn-around time after receipt of the custom program order.
Programming near the end of the device fabrication process can be achieved, for example, by selective ion implantation through the gate electrode to alter selected device threshold voltages, by leaving out selected contact openings and thus not providing electrical contact to selected devices, or straightforwardly by changing the metal pattern so as to bypass selected devices. There are disadvantages, however, to each of these programming methods. Implanting through the gate to control the threshold, for example, requires very high energy implants to implant through both the gate oxide and the thick polycrystalline silicon gate electrode material. Proper control of threshold voltage is difficult to achieve: too high an implant dose can adversely affect the breakdown voltage of the adjacent junctions; a thicker than expected gate electrode blocks too much of the implant and results in an insufficient change in the device threshold voltage. Leaving out selected contact openings requires more layout area since the memory must be designed with each memory cell having its own contact opening; contacts cannot be shared between cells. Similarly, selective metallization takes extra space on the chip as regular metal runs must be interrupted to route around and avoid the unselected contacts.
Accordingly a need existed to develop a MOS ROM structure and method for programming which would overcome the problems of the prior art to provide a MOS ROM which can be programmed late in the device fabrication process.
It is therefore an object of this invention to provide an improved MOS ROM structure.
It is a further object of this invention to provide an improved method for programming a MOS ROM.
It is a still further object of this invention to provide an improved process for programming a MOS ROM which is applicable late in the fabrication sequence.